1. Field of the Invention
The present invention relates to an imageforming apparatus for obtaining a hard copy, such as a copying machine, a facsimile machine and a printer, including an image forming portion which forms a toner image on a sheet utilizing an electrophotographic process by a transfer system or a direct system and a fixing portion which fixes the toner image on the sheet.
2. Description of the Related Art
In an image forming apparatus of an electrophotographic system, when a non-fixed toner image is transferred to a sheet by a transfer portion and this sheet is conveyed to a fixing portion, the sheet is generally conveyed by a conveying belt which is in contact with a non-image surface and the sheet enters a fixing nip portion. At that time, however, especially in the case of a thin media (type of sheet), the sheet is conveyed with a wrinkle or slack in some cases. At the same time, a conveying ability accepting conveyance of a thick sheet is also required, and when a conveying force of a sheet is weak, slip or skew feeding of a sheet may occur.
Further, even if a wrinkle or slack is small, when a sheet tip end is caught in a fixing nip portion, the wrinkle is amplified and becomes a large wrinkle in some cases. It is important to stabilize the pre-fixing sheet conveyance for enhancing quality of a fixing operation to satisfy wide media. The above problem is indicated in the pre-fixing conveying portion, and various techniques for stably conveying sheets are discussed.
As a configuration for preventing slack of a sheet, there is one in which conveying portions including at least two sets of belts are driven at different velocities to prevent the slack of the sheet and as a result, stable conveyance at the pre-fixing conveying portions before the sheet enters the fixing portion is realized (see Japanese Patent Laid-Open No. 2007-033767).
According to this conventional technique, a controlling portion controls a plurality of driving portions such that conveying velocities of both ends in a direction intersecting with a conveying direction of a sheet become faster than a conveying velocity of a central portion of the sheet in a width direction of the sheet. That is, when three driving portions are located in the width direction of the sheet, for example, control is performed such that conveying velocities of the sheets on both ends by the driving portion become faster than the conveying velocity of the sheet by the central driving portion. By making the conveying velocities of both ends of the sheet faster than the conveying velocity of the central portion, it is possible to guide the sheet to a nip portion which is formed by contact between the fixing portion and the driving portions while biasing the sheet such as to widen the sheet in the direction intersecting with the conveying direction.
If the sheet is guided to the fixing nip portion while widening the sheet in the width direction with velocity difference between the driving portions, it is possible to decrease a wrinkle or image failure which may be caused by a passing posture of the sheet, but there is a possibility that the following problem is caused. That is, by making the conveying velocities different from each other in the width direction of the sheet, an effect of smoothing a wrinkle of a sheet can be expected, but due to the velocity difference between the driving portions, there is an adverse possibility that slip or distortion of the sheet is generated between the driving portions having the velocity difference. When the driving portion includes an endless belt for example, it is necessary to complicate or optimize a “vacuum plenum (slip limitation/before fixing/vacuum transfer apparatus)” and a “perforated belt”. It is difficult to accept a plurality of media (surface properties/basis weight/size) and it is necessary to variously change setting conditions.
Hence, the present invention provides an image forming apparatus capable of preventing a sheet deformation such as curl and corrugation while solving the problem of generation of slip or distortion of a sheet caused by velocity difference between the driving portions.